In many applications including ophthalmic surgery, it is necessary to closely and proportionately regulate the pressure (including negative pressure or vacuum) of a controlled volume such as the debris receptacle, which is connected to a probe or cannula used in ophthalmic surgery. Such instruments draw eye tissue into the cutting ridges by the use of suction. Tissue removal rate is directly related to the level of negative pressure or vacuum applied to the instrument. Thus, controlling the negative pressure or vacuum level to a fine degree is an absolute necessity to provide the surgeon a like degree of control of tissue removal. Loss of pressure or vacuum control will result in damage to the eye, e.g. withdrawing or suctioning of unwanted material from the eye's interior. In ophthalmic surgery, the surgeon needs the capability of rapidly decreasing or eliminating the vacuum. A vivid example of this need arises when the surgeon may inadvertently start to cut and suction a part of the eye retina into the surgical instrument. At that instant, the surgeon must immediately remove the vacuum from the probe.
Pressure/vacuum control systems employed in ophthalmic surgery generally include a source of negative pressure or vacuum coupled to the controlled volume or debris receptacle, a fluid control valve disposed therebetween and a surgical instrument connected to the debris receptacle. U.S. Pat. Nos. 4,395,258 and 4,706,687 are illustrative of such prior art systems and both disclose the use of solenoid type valves, which are electrically controlled to be fully opened or fully closed. These systems include a pressure sensor coupled to the debris receptacle for measuring the pressure therein and a feedback type of control system, which responds to a negative pressure less than a preset limit, to open its solenoid valve, whereby the vacuum or negative pressure within the debris receptacle may be increased. A feedback signal is applied to actuate the solenoid valve, which immediately opens to interconnect the debris receptacle to the vacuum source, whereby the pressure within the debris receptacle is immediately reduced.
The system disclosed in U.S. Pat. No. 4,395,258 also includes a second solenoid type valve interconnected between the debris receptacle and atmospheric pressure. In typical operation, each solenoid valve is totally opened or closed. If the vacuum within the debris receptacle is found to be too high, the second valve connected to atmospheric pressure is opened, whereas the first valve remains closed. Oppositely, if the vacuum is determined to be too low, the first valve is opened and the second atmospheric valve is closed. More specifically, these solenoid valves are operated in a pulse modulation (PM) or bang-bang mode whereby the fluid flow therethrough is regulated. In particular, the valve signal is turned on and off so that the duty ratio of off to on permits the desired fluid flow rate. When turned on, the valve signal is applied to the solenoid valve which moves towards its open position to permit fluid flow therethrough and when off, the valve signal is removed from the valve which returns towards its closed position, typically with the aid of a spring bias. Thus, the solenoid valve is rapidly moved between its open and close positions. There are several disadvantages to the use of such solenoid valves and of operating them in a PM mode. First, such valves are relatively noisy. Solenoid valves tend to wear out relatively quickly in that the valve is opening and closing rapidly; the valve seats are repeatedly struck and tend to wear out under repeated impact. Also, such solenoid valves operated in a PM mode tend to respond relatively slowly to a desired change in fluid level, requiring as much as 1-2 seconds to adjust from a first steady state condition for a first fluid flow to a second steady state condition for a second, different fluid flow. Further, the continuous opening and closing of such solenoid valves tends to impart pulsations to the fluid whose pressure level is being regulated. When it is desired to increase the speed with which a pressure change is made, it is necessary to employ a larger solenoid valve having a larger opening and valve mechanism. Though speed may be increased by a larger solenoid valve, larger solenoid valves tend to impart even greater pressure perturbations to the regulated fluid.